a study of cooling system of a spark ignition engine to improve thermal efficiency

7/30/2019 A Study of Cooling System of a Spark Ignition Engine to Improve Thermal Efficiency

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A Study of Cooling System of a Spark Ignition Engine to Improve Thermal Efficiency

Tatsuya Kuboyama1, Yasuo Moriyoshi

1, Mitsuru Iwasaki

2and Junichiro Hara

2

1Department of Artificial Systems Science, Chiba University, Chiba, Japan

(Tel : +81-43-290-3916; E-mail: [email protected])2Calsonic Kansei

Abstract: To improve thermal efficiency of gasoline engines, the effect of coolant system on fuel consumption rate is

studied in this report. The authors tried to control the flow of cooling water to find an optimized flow system to improve

thermal efficiency, especially for the cold start condition. Experiments were carried out using a four-cylinder SI engine.

Three test conditions of i) normal cold start, ii) cold start without operating the water pump and iii) cold start but with

heated engine oil were measured. Temperatures of cooling water at some positions were measured during the

warming-up time. Secondly, the effect of cooling water temperature on thermal efficiency at a steady state condition wasexamined. As a result, a higher temperature causes an improvement of thermal efficiency due to a reduction i n

mechanical friction.

Keywords: Gasoline engine, Thermal efficiency, Cooling system, Cooling water temperature

1. INTRODUCTION

Improvement of thermal efficiency has been strongly

demanded for internal combustion engines. One of themethods to increase theoretical thermal efficiency is an

increase in compression ratio. However, the increase

in compression ratio is limited by knocking. A

reduction in energy loss including cooling loss and

mechanical friction also improves brake thermal

efficiency. It is well known that cooling watertemperature and combustion chamber wall temperature

largely affect cooling loss and knocking. Temperatureof lubricant oil affects mechanical friction. High oil

temperature reduces oil viscosity leading to decreasing

friction loss. In summary, thermal management has

been more important for combustion engines forimproving thermal efficiency. In recent days,

electrically controlled water pump and electric thermo-

stat have been utilized to optimize cooling water

temperature according to operating conditions[1], [2].

It is considered that the development of these

electrically controllable devices allows more precise

thermal management for improvement in performance

of gasoline engines.

In the present study, as a first step to develop asophisticated controlling method for a cooling system of

a gasoline engine, thermal efficiency during cold

start operation and steady state operation were

experimentally investigated using a

commercially available four-cylinder gasoline

engine.

2. TEST ENGINE AND CONDITIONS

Table 1 shows the specifications of the test engine. Acommercially available four-cylinder gasoline engine

was used. The engine has a displacement volume of

1240 cm3, and a compression ratio of 9.8. The ECU

(Electronic control unit) which is normally applied to

production engine was used for controlling of engine

operating parameters, such as an ignition timing and a

fuel injection timing. Figure 1(a) shows cooling water

path and measurement locations of cooling watertemperature for cold start testing. During the warm up

period, thermo-stat closes, and cooling water circulates

through the bypass. After a cooling water temperature

increases, thermo-stat opens, and cooling water

circulates through the radiator which is soaked in the

water tank. Figure 1(b) shows a cooling water pathand measurement locations of wall temperature for

steady state testing. In the steady state testing, thecoolant water path for the cylinder block and that for the

cylinder head are separated. This allows independent

control of temperature and flow rate of cooling water

for the cylinder head and the cylinder block of the testengine.

In this system, temperature in the water tank is

controlled. A cooling water temperature is not directly

controlled but determined by the water tank temperature

and operating conditions. Cooling water temperature

was measured at ten locations shown in figure 1 (Ch 1

Ch 10) during cold start experiment. Additionally,

intake and exhaust gas temperature, cylinder head wall

temperature at the spark plug location was measured.In-cylinder pressure of #3 cylinder was measured by a

piezo-electric pressure transducer (Kister 6117B).

Concentrations of exhaust emissions including CO, O2,

NO, HC, CO2 and mass ratio of air to fuel (A/F) were

measured by an exhaust gas analyzer (HORIBA,

MEXA-584L).

Experiments were carried out during cold start

condition and steady state conditions. During the cold

star testing, the engine operation was started with a

cooling water temperature of 25oC, and the cooling

water temperature and exhaust emissions were

measured for 15 minutes with a sampling rate of 1 Hz.

In-cylinder pressure of #3 cylinder was measured at twotimes, at 4 minutes and 13 minutes after start of engine

SICE Annual Conference 2011September 13-18, 2011, Waseda University, Tokyo, Japan

PR0001/11/0000-0467 400 2011 SICE- 467 -


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operation.

The cold start testing was conducted with threedifferent conditions.

a) Normal cold start operationb) Cold start operation without operating a water

pump. In this case, cooling water did not circulate

during warm up duration.c) Initial temperature of lubricant oil increases up to

100oC.

Table1 Engine specifications

Engine TypeIn-line

4-cylinder

Compression ratio 9.8

Displacement [cc] 310.0

Bore x Stroke [mm] 71.0 x 78.3

Connecting rod length [mm] 120.75

Idle speed [r/min] 700

Thermostat open temperature [oC] 821.5

Thermostat close temperature [oC] 77

In the steady state testing, to investigate the effect of

cooling water temperature on thermal efficiency,

cooling water temperature for the cylinder block and

that for the cylinder head were varied as shown in Table

2. Mass averaged cooling water temperature was fixed

among the three cases. A higher cooling water

temperature for cylinder head would have effect on

indicated thermal efficiency because the cylinder head

wall temperature affects combustion characteristics and

knocking limit. Also, a higher cooling water

temperature for the cylinder block is expected toimprove a brake thermal efficiency by reducing

mechanical friction due to an increase in lubricant oil

temperature. In this experiment, the engine was

running at 2000 rpm. Spark timing was set at the

minimum advance for best torque (MBT). The engine

load was varied from IMEP of 300 kPa to 900 kPa.

Table 2. Experimental conditions

Cylinder block

coolant water

temp. C

Cylinder head

coolant water

temp. C

Case 1 80 C 80 C

Case 2 60 C 100 C

Case 3 100 C 60 C

3. RESULTS AND DISCUSSION

3.1 Effect of cooling system on engine performance

during cold start condition

Figure 2 shows temporal variation in cooling water

temperature during cold start testing.With a normal cold start operation (a), cooling water

temperature increases during warming up operation

(until 590 sec after start of engine operation). This isbecause the thermo-stat does not work during warming

Fig.1(a) Cooling water path and cooling watertemperature measurement locations for cold start

experiment

Figure 1 (b) Cooling water path and wall

temperature measurement locations for steady stateexperiment

up operation. It is found that difference in measured

temperature at location Ch 2 and Ch 8 is 20oC. This

indicates that there is temperature difference between

the cylinders. Cylinder wall temperature distribution

should be reduced to improve total performance of agasoline engine, because engine operating parameters,

such as ignition timing, are determined and restricted by

the worst cylinder. Once a cooling water temperature

increases (after 590 sec. from start of engine operation),temperature of cooling water suddenly decreases. This

is because the thermo-stat begins to work, and relatively

low temperature cooling water is introduced into the

engine cooling path from the radiator. Shortly after the

decrease in cooling water temperature, the thermo-stat

closes again, and cooling water temperature increases.After warming up operation, cooling water temperature

is kept constant.

In case that the water pump is stopped during warmup operation (b), cooling water temperature increases

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even though thermo-stat opens because low temperature

water is not introduced.In case that the initial lubricant oil temperature

increases up to 100oC, temporal variation in cooling

water temperature is similar to the normal cold start

conditions. However, warm up duration decreases by

approximately 95 seconds. High temperature oilwould warm up engine components. This would reducewarm up duration. The similar variation in cooling

water temperature between different measurement

locations to the normal cold start operation is observed

in the case (b) and (c). It could be concluded that the

cooling water temperature distribution during cold start

operation would not be affected by cooling watercirculation and lubricant oil temperature. To realize

uniform water temperature distribution and uniform

cylinder wall temperature distribution, a basic design of

the cooling water path must be optimized.

0

20

40

60

80

100

120

140

0 300 600 900

Time[sec]

Temperature[]

CH1

CH2

CH7

CH8

(a) normal cold start condition

0

20

40

60

80

100

120

140

0 300 600 900

Time[sec]

Temperature[

]

CH1

CH2

CH7

CH8

(b) cold start w/o water pump condition

0

20

40

60

80

100

120

140

0 300 600 900

Time[sec]

Temperature[]

CH1

CH2

CH7CH8

(c) cold start with initially warmed oil

condition

Fig.2 Temporal variations of coolant temp.

Figure 3 shows engine speed, exhaust gas

temperature, lubricant oil temperature during cold start

operation. It can be seen that the difference in the

experimental conditions does not affects engine speed

and exhaust gas temperature. Figure 4 shows fuelconsumption rate and cumulative fuel consumption. The

engine operation started 2 seconds after start of data

acquisition. From these figures, it can be seen that a

large amount of fuel is consumed during first 5 seconds

after start of engine operation. It is also found that the

amount of fuel consumed during cold start with normal

operation (a) is largest among the three cases. Fuel

consumption with high initial oil temperature is lowest.

Fuel consumption during cold start operation could beimproved by 2.6 % with stopping cooling water

circulation, and by 3.5 % with an initial high

temperature lubricant oil.

(a) Engine speed

(b) Exhaust gas temperature

(c) Lubricant oil temperatureFig.3 Temporal variation engine speed, exhaust

gas temperature and lubricant oil temperature

for different cold start conditions

3.2 Effect of cooling system on engine performance

during steady state operation

Figure 5 shows effect of cooling water temperature

on cylinder head wall temperature. It is found that

cylinder head temperature increases with increasingcoolant water temperature on cylinder head. Also, it

0

200

400

600

800

1000

1200

1400

1600

1800

0 300 600 900

Time[sec]

Enginespeed[r/min]

Normal

Without belt

Hot oil start

0

50

100

150

200

250

300

350

400

0 300 600 900

Time[sec]

Temperature[]

Normal

Without belt

Hot oil start

0

10

20

30

40

50

60

70

0 300 600 900

Time[sec]

Temperature[]

Normal

Without belt

Hot oil start

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can be seen that the exhaust side wall temperature is

more sensitive to the cooling water temperature.Figure 6 shows effect of cooling water temperature on

thermal efficiency. Indicated thermal efficiency was

not affected by the cooling water temperature.

However, brake thermal efficiency could be improved

by increasing the cooling water temperature for thecylinder block. Brake thermal efficiency with Case 3(Block 100

oC) was improved by 3 to 5%, compared to

Case 1(Block 80oC). This is probably because of a

reduction in mechanical friction due to an increase in

lubricant oil temperature with the increase in a cooling

water temperature for the cylinder block. From the

results obtained in this study suggest that thermalefficiency could be improved by appropriate controlling

of cooling water temperature depending on the

operational conditions. As a next step, the authors will

try to develop a controlling strategy of cooling water

temperature for improving a thermal efficiency.

(a) Fuel mass flow rate

(b) Fuel mass flow rate (expanded)

(c) Cumulative fuel consumption during coldstart oeration

Fig.4 Effect of cold start conditions on fuelconsumption

Fig. 5 The effect of coolant water temperature oncylinder head wall temperature

(upper: intake side, lower: exhaust side)

Fig.6. The effect of coolant water temperature on

thermal efficiency and mechanical loss

0

0.5

1

1.5

2

2.5

0 300 600 900

Time[sec]

Gasolinemassflow

rate[g/s]

Normal

Without belt

Hot oil start

0

0.5

1

1.5

2

2.5

0 5 10 15 20

Time[sec]

Gasolinemassflow

rate[g/s]

Normal

Without belt

Hot oil start

0

2

4

6

8

10

12

0 5 10 15 20

Time[sec]

Gasolinemassflow

rate[g]

Normal

Without belt

Hot oil start

80

90

100

110

120

130

140

150

160

#1 #2 #3 #4

Walltemp.

(Intakeside)oC

Cylinder number

Head 80/Block 80

Head 100/Block 60

Head 60/Block 100

80

90

100

110

120

130

140

150

160

#1 #2 #3 #4

Walltempe.(Exh.side)oC

Cylinder number

Head 80/Block 80Head 100/Block 60

Head 60/Block 100

15

20

25

30

35

40

0 200 400 600 800 1000Brakethermalefficiency%

IMEP kPa

Head 60/Block 100

Head 80/Block 80

Head 100/Block 60

15

20

25

30

35

40

0 200 400 600 800 1000

Brakethermalefficiency%

IMEP kPa

Head 60/Block 100

Head 80/Block 80

Head 100/Block 60

10

15

20

25

30

35

40

0 200 400 600 800 1000

Mechanicalloss%

IMEP kPa

Head 60/Block 100

Head 80/Block 80

Head 100/Block 60

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4. SUMMARY

As a first step of developing a control strategy of

cooling system of gasoline engines for improvement in

thermal efficiency, thermal efficiency during cold start

operation and steady state operation were

experimentally investigated using a commerciallyavailable four-cylinder gasoline engine. Experimental

results can be summarized as follows;

(1) Fuel consumption during cold start operation couldbe improved by 2.6 % with stopping cooling water

circulation, and by 3.5 % with an initial high

temperature lubricant oil.

(2) During cold start operation, a spatial distribution ofcooling water temperature which would result in

wall temperature variation between the cylinder

was not affected by oil temperature and cooling

water circulation. This indicates that a basicdesign of the cooling water path is dominant factor

for the spatial distribution of a cooling water

temperature.

(3) A higher cooling water temperature for the cylinderblock could improve brake thermal efficiencyduring steady state operation because of a reduction

in mechanical friction. With the increase in

cooling water temperature for the cylinder block

from 60oC to 100

oC, break thermal efficiency was

improved by 3 to 5%.

ACKNOWLEDGEMENT

The authors would like to thankKistler Japan, Co.

ltd., for their support in the pressure

measurements. Mr. S, Notoya and K. Toida of

Chiba University are also acknowledged for theirsupport in the experimental works.

REFERENCES

[1] E.Sonntag et al., The New Family of Small 4

-Cylinder Engines, JSAE paper No.20075365, 2007.[2] A. Eiser et al., The new 1.8L TFSI engine from

Audi, Part 1 Base engine and Thermo-management,

MTZ vol. 72, pp. 23- 39, 2011.

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a study of cooling system of a spark ignition engine to improve thermal efficiency

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